CHEMCATCHEM
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cation in concentrated sulfuric acid (g=2.002583). Nitroxide con-
centrations were measured with respect to a solution of DPPH of
known concentration by using the signal from a ruby crystal as an
internal standard.[25] The electrochemical cell was home-made and
consisted of an EPR flat cell (Wilmad WG-810) equipped with
a 25ꢁ5ꢁ0.2 mm platinum gauze (cathode) and a platinum wire
(anode). The current was supplied and controlled by an AMEL 2051
general-purpose potentiostat. In a typical experiment, the cell was
filled with an acetonitrile solution obtained after filtration of a sus-
pension of the catalytic systems employed for 12 cycles in ACN
(5 mgmLÀ1) containing tetrabutylammonium perchlorate (ꢀ0.1m)
as supporting electrolyte. After thoroughly purging the solution
with N2, spectra were recorded at a potential settings of À3.0 V.[26]
Figure 7. EPR spectra of 3a recorded at different times in ACN, containing
Bu4NClO4 (ꢀ0.1m) as supporting electrolyte, at a potential setting of À3.0 V.
The starting solution was obtained after filtration of a suspension of the cat-
alytic systems 3a/4 employed for 12 cycles in ACN.
Synthesis of TEMPO derivatives 2a and 2b
To a solution of 4-hydroxy-TEMPO (2.32 mmol, 400 mg) in toluene
(4 mL), Bu4NBr (0.124 mmol, 40 mg) was added followed by 1,3,5-
tri(bromomethyl)mesitylene 1a (4.64 mmol, 1.85 g) and NaOH 50%
(8 mL). The mixture was heated at 708C for 2 h. After cooling at RT,
toluene and water were added. The aqueous layer was separated
and extracted with ethyl acetate (3ꢁ), then organic layers were
mixed and evaporated. The residue was purified by column chro-
matography by using dichloromethane/petroleum ether 7:3 to re-
cover the unreacted tribromo compound 1a, whereas the desired
compound 2a was eluted with dichloromethane (60% yield).[27]
ing the process in a homogeneous fashion. After solvent re-
moval, the catalyst was readsorbed allowing an easy recovery
and recycle of the catalytic material up to 13 consecutive
cycles with no loss in activity. Such adsorption–desorption
equilibrium has been unequivocally proven by EPR experi-
ments, which further confirmed that the catalyst was in its ox-
oammonium salt form after the catalytic cycle.
The catalytic system could be employed in low catalytic
amount (1 mol%), and resulted to be more recyclable if ad-
sorbed on imidazolium-modified silica gel. Higher loading
(10 mol%) both on silica gel or imidazolium-modified silica gel
gave highly recyclable materials. Thanks to the high recyclabili-
ty of the system, the use of such high amounts does not repre-
sent, in our opinion, a limitation of the method.
Compound 2b was prepared by following the same procedure
(63% yield).
Synthesis of catalysts 3a and 3b
In summary, our system appears to be more recyclable with
respect to other IL–TEMPO catalysts reported in the literature,
although improvements are desirable. Further studies are in
progress to avoid the use of stoichiometric amounts of the
co-oxidant.
A solution of compound 2a (1.13 mmol, 0.55 g) and methylimid-
azole (2.49 mmol, 0.197 mL) in CHCl3 (13 mL) was heated at 608C
overnight. After cooling at RT, the solvent was removed and the
solid residue was carefully washed with hot diethyl ether and fil-
tered to give compound 3a as a light orange solid (89% yield).
TEMPO catalyst 3b was obtained as reddish viscous oil (99% yield).
Experimental Section
Alcohol oxidation
General
General procedure: Catalyst 3a or 3b (10 mol%) were dissolved in
methanol (4 mL) in a round-bottom flask and support 4 (loading
25 wt%: 212 mg for catalyst 3a, 200 mg for catalyst 3b; loading
12.5 wt%: 424 mg for catalyst 3a, 400 mg for catalyst 3b) was
added. The solvent was removed under reduced pressure and
a powder 3a/4 or 3b/4 was obtained. Then, dichloromethane
(2 mL) was added and, to this suspension, the appropriate alcohol
(0.82 mmol) and BAIB (1.1 equiv.) were added. The mixture was
stirred at RT for the time indicated in the tables, then the solvent
was removed under reduced pressure and the residue washed
with diethyl ether and filtered. The catalytic material was dried for
a few minutes and then reused for the next cycle.
Chemicals and solvents were purchased from commercial suppliers
or purified by standard techniques. For thin-layer chromatography
(TLC), silica gel plates (Merck 60 F254) were used and compounds
were visualized by irradiation with UV light and/or by treatment
with a solution of KMnO4. Flash chromatography was performed
by using Macherey–Nagel silica gel (0.04–0.063 mm). Light petro-
1
leum refers to the fraction with the boiling range 40–608C. H and
13C NMR spectra were recorded with a Bruker 300 MHz spectrome-
ter. SEM images were recorded by using an instrument with 20 kV
operating voltage. Support 4 was prepared as described in the lit-
erature.[21] Silica gel used as a support has a surface area of
750 m2 gÀ1 and a cumulative pore volume of 0.68 cm3 gÀ1 and is
commercially available (Aldrich).
Acknowledgements
EPR experiments
Financial support from the University of Palermo (funds for se-
lected topics), University of Bologna and the Italian MIUR within
the national project “Catalizzatori, metodologie e processi inno-
vativi per il regio e stereocontrollo delle sintesi organiche” and
COST Action CM0905 ORCA are gratefully acknowledged.
EPR spectra were recorded at RT using an ELEXYS E500 spectrome-
ter equipped with an NMR gaussmeter for the calibration of the
magnetic field and a frequency counter for the determination of g
factors that were corrected against that of the perylene radical
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